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Is this intentional? -- Found something which looks like a code comment -- if you have code sections in the document, please surround them with '' and '' lines. Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Missing Reference: 'CFWS' is mentioned on line 496, but not defined == Missing Reference: 'RFC7208' is mentioned on line 804, but not defined ** Obsolete normative reference: RFC 5451 (Obsoleted by RFC 7001) ** Obsolete normative reference: RFC 6577 (Obsoleted by RFC 7001) ** Obsolete normative reference: RFC 7001 (Obsoleted by RFC 7601) ** Obsolete normative reference: RFC 7601 (Obsoleted by RFC 8601) -- Obsolete informational reference (is this intentional?): RFC 4870 (ref. 'DOMAINKEYS') (Obsoleted by RFC 4871) -- Obsolete informational reference (is this intentional?): RFC 3501 (ref. 'IMAP') (Obsoleted by RFC 9051) -- Obsolete informational reference (is this intentional?): RFC 7410 (Obsoleted by RFC 7601) Summary: 4 errors (**), 0 flaws (~~), 3 warnings (==), 6 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 DMARC Working Group M. Kucherawy 3 Internet-Draft February 7, 2018 4 Obsoletes: 7601 (if approved) 5 Intended status: Standards Track 6 Expires: August 11, 2018 8 Message Header Field for Indicating Message Authentication Status 9 draft-ietf-dmarc-rfc7601bis-00 11 Abstract 13 This document specifies a message header field called Authentication- 14 Results for use with electronic mail messages to indicate the results 15 of message authentication efforts. Any receiver-side software, such 16 as mail filters or Mail User Agents (MUAs), can use this header field 17 to relay that information in a convenient and meaningful way to users 18 or to make sorting and filtering decisions. 20 Status of this Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on August 11, 2018. 37 Copyright Notice 39 Copyright (c) 2018 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 55 1.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 5 56 1.2. Trust Boundary . . . . . . . . . . . . . . . . . . . . . . 6 57 1.3. Processing Scope . . . . . . . . . . . . . . . . . . . . . 6 58 1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 6 59 1.5. Definitions . . . . . . . . . . . . . . . . . . . . . . . 7 60 1.5.1. Key Words . . . . . . . . . . . . . . . . . . . . . . 7 61 1.5.2. Security . . . . . . . . . . . . . . . . . . . . . . . 7 62 1.5.3. Email Architecture . . . . . . . . . . . . . . . . . . 8 63 1.5.4. Other Terms . . . . . . . . . . . . . . . . . . . . . 9 64 1.6. Trust Environment . . . . . . . . . . . . . . . . . . . . 9 65 2. Definition and Format of the Header Field . . . . . . . . . . 9 66 2.1. General Description . . . . . . . . . . . . . . . . . . . 9 67 2.2. Formal Definition . . . . . . . . . . . . . . . . . . . . 10 68 2.3. Property Types (ptypes) and Properties . . . . . . . . . . 12 69 2.4. The "policy" ptype . . . . . . . . . . . . . . . . . . . . 13 70 2.5. Authentication Identifier Field . . . . . . . . . . . . . 14 71 2.6. Version Tokens . . . . . . . . . . . . . . . . . . . . . . 15 72 2.7. Defined Methods and Result Values . . . . . . . . . . . . 16 73 2.7.1. DKIM and DomainKeys . . . . . . . . . . . . . . . . . 16 74 2.7.2. SPF and Sender ID . . . . . . . . . . . . . . . . . . 17 75 2.7.3. "iprev" . . . . . . . . . . . . . . . . . . . . . . . 19 76 2.7.4. SMTP AUTH . . . . . . . . . . . . . . . . . . . . . . 19 77 2.7.5. Other Registered Codes . . . . . . . . . . . . . . . . 21 78 2.7.6. Extension Methods . . . . . . . . . . . . . . . . . . 21 79 2.7.7. Extension Result Codes . . . . . . . . . . . . . . . . 22 80 3. The "iprev" Authentication Method . . . . . . . . . . . . . . 22 81 4. Adding the Header Field to a Message . . . . . . . . . . . . . 23 82 4.1. Header Field Position and Interpretation . . . . . . . . . 25 83 4.2. Local Policy Enforcement . . . . . . . . . . . . . . . . . 26 84 5. Removing Existing Header Fields . . . . . . . . . . . . . . . 26 85 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27 86 6.1. The Authentication-Results Header Field . . . . . . . . . 27 87 6.2. "Email Authentication Methods" Registry Description . . . 28 88 6.3. "Email Authentication Methods" Registry Update . . . . . . 28 89 6.4. "Email Authentication Property Types" Registry . . . . . . 28 90 6.5. "Email Authentication Result Names" Description . . . . . 28 91 6.6. "Email Authentication Result Names" Update . . . . . . . . 28 92 7. Security Considerations . . . . . . . . . . . . . . . . . . . 28 93 7.1. Forged Header Fields . . . . . . . . . . . . . . . . . . . 28 94 7.2. Misleading Results . . . . . . . . . . . . . . . . . . . . 30 95 7.3. Header Field Position . . . . . . . . . . . . . . . . . . 31 96 7.4. Reverse IP Query Denial-of-Service Attacks . . . . . . . . 31 97 7.5. Mitigation of Backscatter . . . . . . . . . . . . . . . . 31 98 7.6. Internal MTA Lists . . . . . . . . . . . . . . . . . . . . 31 99 7.7. Attacks against Authentication Methods . . . . . . . . . . 31 100 7.8. Intentionally Malformed Header Fields . . . . . . . . . . 32 101 7.9. Compromised Internal Hosts . . . . . . . . . . . . . . . . 32 102 7.10. Encapsulated Instances . . . . . . . . . . . . . . . . . . 32 103 7.11. Reverse Mapping . . . . . . . . . . . . . . . . . . . . . 32 104 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33 105 8.1. Normative References . . . . . . . . . . . . . . . . . . . 33 106 8.2. Informative References . . . . . . . . . . . . . . . . . . 34 107 Appendix A. Legacy MUAs . . . . . . . . . . . . . . . . . . . . . 36 108 Appendix B. Authentication-Results Examples . . . . . . . . . . . 37 109 B.1. Trivial Case; Header Field Not Present . . . . . . . . . . 37 110 B.2. Nearly Trivial Case; Service Provided, but No 111 Authentication Done . . . . . . . . . . . . . . . . . . . 38 112 B.3. Service Provided, Authentication Done . . . . . . . . . . 39 113 B.4. Service Provided, Several Authentications Done, Single 114 MTA . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 115 B.5. Service Provided, Several Authentications Done, 116 Different MTAs . . . . . . . . . . . . . . . . . . . . . . 41 117 B.6. Service Provided, Multi-tiered Authentication Done . . . . 43 118 B.7. Comment-Heavy Example . . . . . . . . . . . . . . . . . . 44 119 Appendix C. Operational Considerations about Message 120 Authentication . . . . . . . . . . . . . . . . . . . 45 121 Appendix D. Changes since RFC 7001 . . . . . . . . . . . . . . . 46 122 Appendix E. Acknowledgments . . . . . . . . . . . . . . . . . . . 48 123 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 48 125 1. Introduction 127 This document describes a header field called Authentication-Results 128 for electronic mail messages that presents the results of a message 129 authentication effort in a machine-readable format. The intent of 130 the header field is to create a place to collect such data when 131 message authentication mechanisms are in use so that a Mail User 132 Agent (MUA) and downstream filters can make filtering decisions 133 and/or provide a recommendation to the user as to the validity of the 134 message's origin and possibly the safety and integrity of its 135 content. 137 End users are not expected to be direct consumers of this header 138 field. This header field is intended for consumption by programs 139 that will then use such data or render it in a human-usable form. 141 This document specifies the format of this header field and discusses 142 the implications of its presence or absence. However, it does not 143 discuss how the data contained in the header field ought to be used, 144 such as what filtering decisions are appropriate or how an MUA might 145 render those results, as these are local policy and/or user interface 146 design questions that are not appropriate for this document. 148 At the time of publication of this document, the following are 149 published email authentication methods: 151 o Author Domain Signing Practices ([ADSP]) (Historic) 153 o SMTP Service Extension for Authentication ([AUTH]) 155 o DomainKeys Identified Mail Signatures ([DKIM]) 157 o Domain-based Message Authentication, Reporting and Conformance 158 ([DMARC]) 160 o Sender Policy Framework ([SPF]) 162 o reverse IP address name validation ("iprev", defined in Section 3) 164 o Require-Recipient-Valid-Since Header Field and SMTP Service 165 Extension ([RRVS]) 167 o S/MIME Signature Verification ([SMIME-REG]) 169 o Vouch By Reference ([VBR]) 171 o DomainKeys ([DOMAINKEYS]) (Historic) 172 o Sender ID ([SENDERID]) (Experimental) 174 There exist registries for tokens used within this header field that 175 refer to the specifications listed above. Section 6 describes the 176 registries and their contents and specifies the process by which 177 entries are added or updated. It also updates the existing contents 178 to match the current states of these specifications. 180 This specification is not intended to be restricted to domain-based 181 authentication schemes, but the existing schemes in that family have 182 proven to be a good starting point for implementations. The goal is 183 to give current and future authentication schemes a common framework 184 within which to deliver their results to downstream agents and 185 discourage the creation of unique header fields for each. 187 Although SPF defined a header field called "Received-SPF" and the 188 historic DomainKeys defined one called "DomainKey-Status" for this 189 purpose, those header fields are specific to the conveyance of their 190 respective results only and thus are insufficient to satisfy the 191 requirements enumerated below. In addition, many SPF implementations 192 have adopted the header field specified here at least as an option, 193 and DomainKeys has been obsoleted by DKIM. 195 1.1. Purpose 197 The header field defined in this document is expected to serve 198 several purposes: 200 1. Convey the results of various message authentication checks, 201 which are applied by upstream filters and Mail Transfer Agents 202 (MTAs) and then passed to MUAs and downstream filters within the 203 same "trust domain". Such agents might wish to render those 204 results to end users or to use those data to apply more or less 205 stringent content checks based on authentication results; 207 2. Provide a common location within a message for this data; 209 3. Create an extensible framework for reporting new authentication 210 methods as they emerge. 212 In particular, the mere presence of this header field does not mean 213 its contents are valid. Rather, the header field is reporting 214 assertions made by one or more authentication schemes (supposedly) 215 applied somewhere upstream. For an MUA or downstream filter to treat 216 the assertions as actually valid, there must be an assessment of the 217 trust relationship among such agents, the validating MTA, and the 218 mechanism for conveying the information. 220 1.2. Trust Boundary 222 This document makes several references to the "trust boundary" of an 223 administrative management domain (ADMD). Given the diversity among 224 existing mail environments, a precise definition of this term isn't 225 possible. 227 Simply put, a transfer from the producer of the header field to the 228 consumer must occur within a context that permits the consumer to 229 treat assertions by the producer as being reliable and accurate 230 (trustworthy). How this trust is obtained is outside the scope of 231 this document. It is entirely a local matter. 233 Thus, this document defines a "trust boundary" as the delineation 234 between "external" and "internal" entities. Services that are 235 internal -- within the trust boundary -- are provided by the ADMD's 236 infrastructure for its users. Those that are external are outside of 237 the authority of the ADMD. By this definition, hosts that are within 238 a trust boundary are subject to the ADMD's authority and policies, 239 independent of their physical placement or their physical operation. 240 For example, a host within a trust boundary might actually be 241 operated by a remote service provider and reside physically within 242 its data center. 244 It is possible for a message to be evaluated inside a trust boundary 245 but then depart and re-enter the trust boundary. An example might be 246 a forwarded message such as a message/rfc822 attachment (see 247 Multipurpose Internet Mail Extensions [MIME]) or one that is part of 248 a multipart/digest. The details reported by this field cannot be 249 trusted in that case. Thus, this field found within one of those 250 media types is typically ignored. 252 1.3. Processing Scope 254 The content of this header field is meant to convey to message 255 consumers that authentication work on the message was already done 256 within its trust boundary, and those results are being presented. It 257 is not intended to provide message parameters to consumers so that 258 they can perform authentication protocols on their own. 260 1.4. Requirements 262 This document establishes no new requirements on existing protocols 263 or servers. 265 In particular, this document establishes no requirement on MTAs to 266 reject or filter arriving messages that do not pass authentication 267 checks. The data conveyed by the specified header field's contents 268 are for the information of MUAs and filters and are to be used at 269 their discretion. 271 1.5. Definitions 273 This section defines various terms used throughout this document. 275 1.5.1. Key Words 277 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 278 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 279 document are to be interpreted as described in [KEYWORDS]. 281 1.5.2. Security 283 "Guidelines for Writing RFC Text on Security Considerations" 284 ([SECURITY]) discusses authentication and authorization and the 285 conflation of the two concepts. The use of those terms within the 286 context of recent message security work has given rise to slightly 287 different definitions, and this document reflects those current 288 usages, as follows: 290 o "Authorization" is the establishment of permission to use a 291 resource or represent an identity. In this context, authorization 292 indicates that a message from a particular ADMD arrived via a 293 route the ADMD has explicitly approved. 295 o "Authentication" is the assertion of validity of a piece of data 296 about a message (such as the sender's identity) or the message in 297 its entirety. 299 As examples: SPF and Sender ID are authorization mechanisms in that 300 they express a result that shows whether or not the ADMD that 301 apparently sent the message has explicitly authorized the connecting 302 Simple Mail Transfer Protocol ([SMTP]) client to relay messages on 303 its behalf, but they do not actually validate any other property of 304 the message itself. By contrast, DKIM is agnostic as to the routing 305 of a message but uses cryptographic signatures to authenticate 306 agents, assign (some) responsibility for the message (which implies 307 authorization), and ensure that the listed portions of the message 308 were not modified in transit. Since the signatures are not tied to 309 SMTP connections, they can be added by either the ADMD of origin, 310 intermediate ADMDs (such as a mailing list server), other handling 311 agents, or any combination. 313 Rather than create a separate header field for each class of 314 solution, this proposal groups them both into a single header field. 316 1.5.3. Email Architecture 318 o A "border MTA" is an MTA that acts as a gateway between the 319 general Internet and the users within an organizational boundary. 320 (See also Section 1.2.) 322 o A "delivery MTA" (or Mail Delivery Agent or MDA) is an MTA that 323 actually enacts delivery of a message to a user's inbox or other 324 final delivery. 326 o An "intermediate MTA" is any MTA that is not a delivery MTA and is 327 also not the first MTA to handle the message. 329 The following diagram illustrates the flow of mail among these 330 defined components. See Internet Mail Architecture [EMAIL-ARCH] for 331 further discussion on general email system architecture, which 332 includes detailed descriptions of these components, and Appendix C of 333 this document for discussion about the common aspects of email 334 authentication in current environments. 336 +-----+ +-----+ +------------+ 337 | MUA |-->| MSA |-->| Border MTA | 338 +-----+ +-----+ +------------+ 339 | 340 | 341 V 342 +----------+ 343 | Internet | 344 +----------+ 345 | 346 | 347 V 348 +-----+ +-----+ +------------------+ +------------+ 349 | MUA |<--| MDA |<--| Intermediate MTA |<--| Border MTA | 350 +-----+ +-----+ +------------------+ +------------+ 352 Generally, it is assumed that the work of applying message 353 authentication schemes takes place at a border MTA or a delivery MTA. 354 This specification is written with that assumption in mind. However, 355 there are some sites at which the entire mail infrastructure consists 356 of a single host. In such cases, such terms as "border MTA" and 357 "delivery MTA" might well apply to the same machine or even the very 358 same agent. It is also possible that some message authentication 359 tests could take place on an intermediate MTA. Although this 360 document doesn't specifically describe such cases, they are not meant 361 to be excluded. 363 1.5.4. Other Terms 365 In this document, the term "producer" refers to any component that 366 adds this header field to messages it is handling, and "consumer" 367 refers to any component that identifies, extracts, and parses the 368 header field to use as part of a handling decision. 370 1.6. Trust Environment 372 This header field permits one or more message validation mechanisms 373 to communicate output to one or more separate assessment mechanisms. 374 These mechanisms operate within a unified trust boundary that defines 375 an Administrative Management Domain (ADMD). An ADMD contains one or 376 more entities that perform validation and generate the header field 377 and one or more that consume it for some type of assessment. The 378 field often contains no integrity or validation mechanism of its own, 379 so its presence must be trusted implicitly. Hence, valid use of the 380 header field requires removing any occurrences of it that are present 381 when the message enters the ADMD. This ensures that later 382 occurrences have been added within the trust boundary of the ADMD. 384 The authserv-id token defined in Section 2.2 can be used to reference 385 an entire ADMD or a specific validation engine within an ADMD. 386 Although the labeling scheme is left as an operational choice, some 387 guidance for selecting a token is provided in later sections of this 388 document. 390 2. Definition and Format of the Header Field 392 This section gives a general overview of the format of the header 393 field being defined and then provides more formal specification. 395 2.1. General Description 397 The header field specified here is called Authentication-Results. It 398 is a Structured Header Field as defined in Internet Message Format 399 ([MAIL]), and thus all of the related definitions in that document 400 apply. 402 This header field is added at the top of the message as it transits 403 MTAs that do authentication checks, so some idea of how far away the 404 checks were done can be inferred. It is therefore considered to be a 405 trace field as defined in [MAIL], and thus all of the related 406 definitions in that document apply. 408 The value of the header field (after removing comments) consists of 409 an authentication identifier, an optional version, and then a series 410 of statements and supporting data. The statements are of the form 411 "method=result" and indicate which authentication method(s) were 412 applied and their respective results. For each such statement, the 413 supporting data can include a "reason" string and one or more 414 "property=value" statements indicating which message properties were 415 evaluated to reach that conclusion. 417 The header field can appear more than once in a single message, more 418 than one result can be represented in a single header field, or a 419 combination of these can be applied. 421 2.2. Formal Definition 423 Formally, the header field is specified as follows using Augmented 424 Backus-Naur Form ([ABNF]): 426 authres-header-field = "Authentication-Results:" authres-payload 428 authres-payload = [CFWS] authserv-id 429 [ CFWS authres-version ] 430 ( no-result / 1*resinfo ) [CFWS] CRLF 432 authserv-id = value 433 ; see below for a description of this element 435 authres-version = 1*DIGIT [CFWS] 436 ; indicates which version of this specification is in use; 437 ; this specification is version "1", and the absence of a 438 ; version implies this version of the specification 440 no-result = [CFWS] ";" [CFWS] "none" 441 ; the special case of "none" is used to indicate that no 442 ; message authentication was performed 444 resinfo = [CFWS] ";" methodspec [ CFWS reasonspec ] 445 *( CFWS propspec ) 447 methodspec = [CFWS] method [CFWS] "=" [CFWS] result 448 ; indicates which authentication method was evaluated 449 ; and what its output was 451 reasonspec = "reason" [CFWS] "=" [CFWS] value 452 ; a free-form comment on the reason the given result 453 ; was returned 455 propspec = ptype [CFWS] "." [CFWS] property [CFWS] "=" pvalue 456 ; an indication of which properties of the message 457 ; were evaluated by the authentication scheme being 458 ; applied to yield the reported result 460 method = Keyword [ [CFWS] "/" [CFWS] method-version ] 461 ; a method indicates which method's result is 462 ; represented by "result", and is one of the methods 463 ; explicitly defined as valid in this document 464 ; or is an extension method as defined below 466 method-version = 1*DIGIT [CFWS] 467 ; indicates which version of the method specification is 468 ; in use, corresponding to the matching entry in the IANA 469 ; "Email Authentication Methods" registry; a value of "1" 470 ; is assumed if this version string is absent 472 result = Keyword 473 ; indicates the results of the attempt to authenticate 474 ; the message; see below for details 476 ptype = Keyword 477 ; indicates whether the property being evaluated was 478 ; a parameter to an [SMTP] command, was a value taken 479 ; from a message header field, was some property of 480 ; the message body, or was some other property evaluated by 481 ; the receiving MTA; expected to be one of the "property 482 ; types" explicitly defined as valid, or an extension 483 ; ptype, as defined below 485 property = special-smtp-verb / Keyword 486 ; indicates more specifically than "ptype" what the 487 ; source of the evaluated property is; the exact meaning 488 ; is specific to the method whose result is being reported 489 ; and is defined more clearly below 491 special-smtp-verb = "mailfrom" / "rcptto" 492 ; special cases of [SMTP] commands that are made up 493 ; of multiple words 495 pvalue = [CFWS] ( value / [ [ local-part ] "@" ] domain-name ) 496 [CFWS] 497 ; the value extracted from the message property defined 498 ; by the "ptype.property" construction 500 "local-part" is defined in Section 3.4.1 of [MAIL], and "CFWS" is 501 defined in Section 3.2.2 of [MAIL]. 503 "Keyword" is defined in Section 4.1.2 of [SMTP]. 505 The "value" is as defined in Section 5.1 of [MIME]. 507 The "domain-name" is as defined in Section 3.5 of [DKIM]. 509 The "Keyword" used in "result" above is further constrained by the 510 necessity of being enumerated in Section 2.7. 512 See Section 2.5 for a description of the authserv-id element. 514 If the value portion of a "pvalue" construction identifies something 515 intended to be an email identity, then it MUST use the right hand 516 portion of that ABNF definition. 518 The list of commands eligible for use with the "smtp" ptype can be 519 found in Section 4.1 of [SMTP]. 521 The "propspec" may be omitted if, for example, the method was unable 522 to extract any properties to do its evaluation yet has a result to 523 report. 525 Where an SMTP command name is being reported as a "property", the 526 agent generating the header field represents that command by 527 converting it to lowercase and dropping any spaces (e.g., "MAIL FROM" 528 becomes "mailfrom", "RCPT TO" becomes "rcptto", etc.). 530 A "ptype" value of "policy" indicates a policy decision about the 531 message not specific to a property of the message that could be 532 extracted. See Section 2.4 for details. 534 Examples of complete messages using this header field can be found in 535 Appendix B. 537 2.3. Property Types (ptypes) and Properties 539 The "ptype" in the ABNF above indicates the general type of property 540 being described by the result being reported, upon which the reported 541 result was based. Coupled with the "property", which is more 542 specific, they indicate from where the reported data were extracted. 543 This can include part of the message header or body, some part of the 544 SMTP session, a secondary output of an authentication method (apart 545 from its pure result), or some other aspect of the message's 546 handling. 548 Combinations of ptypes and properties are registered and described in 549 the "Email Authentication Methods" registry, coupled with the 550 authentication methods with which they are used. This is further 551 described in Section 6. 553 Legal values of "ptype" are as defined in the IANA "Email 554 Authentication Property Types" registry, created by [RFC7410]. The 555 initial values and what they typically indicate are as follows, based 556 on [RFC7001]: 558 body: Information that was extracted from the body of the message. 559 This might be an arbitrary string of bytes, a hash of a string of 560 bytes, a Uniform Resource Identifier, or some other content of 561 interest. The "property" is an indication of where within the 562 message body the extracted content was found, and can indicate an 563 offset, identify a MIME part, etc. 565 header: Indicates information that was extracted from the header of 566 the message. This might be the value of a header field or some 567 portion of a header field. The "property" gives a more precise 568 indication of the place in the header from which the extraction 569 took place. 571 policy: A local policy mechanism was applied that augments or 572 overrides the result returned by the authentication mechanism. 573 (See Section 2.4.) 575 smtp: Indicates information that was extracted from an SMTP command 576 that was used to relay the message. The "property" indicates 577 which SMTP command included the extracted content as a parameter. 579 Results reported using unknown ptypes MUST NOT be used in making 580 handling decisions. They can be safely ignored by consumers. 582 Entries in the "Email Authentication Methods" registry can define 583 properties that deviate from these definitions when appropriate. 584 Such deviations need to be clear in the registry and/or in the 585 defining document. See Section 2.7.1 for an example. 587 2.4. The "policy" ptype 589 A special ptype value of "policy" is also defined. This ptype is 590 provided to indicate that some local policy mechanism was applied 591 that augments or even replaces (i.e., overrides) the result returned 592 by the authentication mechanism. The property and value in this case 593 identify the local policy that was applied and the result it 594 returned. 596 For example, a DKIM signature is not required to include the Subject 597 header field in the set of fields that are signed. An ADMD receiving 598 such a message might decide that such a signature is unacceptable, 599 even if it passes, because the content of the Subject header field 600 could be altered post-signing without invalidating the signature. 601 Such an ADMD could replace the DKIM "pass" result with a "policy" 602 result and then also include the following in the corresponding 603 Authentication-Result field: 605 ... dkim=fail policy.dkim-rules=unsigned-subject ... 607 In this case, the property is "dkim-rules", indicating some local 608 check by that name took place and that check returned a result of 609 "unsigned-subject". These are arbitrary names selected by (and 610 presumably used within) the ADMD making use of them, so they are not 611 normally registered with IANA or otherwise specified apart from 612 setting syntax restrictions that allow for easy parsing within the 613 rest of the header field. 615 This ptype existed in the original specification for this header 616 field, but without a complete description or example of intended use. 617 As a result, it has not seen any practical use to date that matches 618 its intended purpose. These added details are provided to guide 619 implementers toward proper use. 621 2.5. Authentication Identifier Field 623 Every Authentication-Results header field has an authentication 624 service identifier field (authserv-id above). Specifically, this is 625 any string intended to identify the authentication service within the 626 ADMD that conducted authentication checks on the message. This 627 identifier is intended to be machine-readable and not necessarily 628 meaningful to users. 630 Since agents consuming this field will use this identifier to 631 determine whether its contents are of interest (and are safe to use), 632 the uniqueness of the identifier MUST be guaranteed by the ADMD that 633 generates it and MUST pertain to that ADMD. MUAs or downstream 634 filters SHOULD use this identifier to determine whether or not the 635 data contained in an Authentication-Results header field ought to be 636 used or ignored. 638 For simplicity and scalability, the authentication service identifier 639 SHOULD be a common token used throughout the ADMD. Common practice 640 is to use the DNS domain name used by or within that ADMD, sometimes 641 called the "organizational domain", but this is not strictly 642 necessary. 644 For tracing and debugging purposes, the authentication identifier can 645 instead be the specific hostname of the MTA performing the 646 authentication check whose result is being reported. Moreover, some 647 implementations define a substructure to the identifier; these are 648 outside of the scope of this specification. 650 Note, however, that using a local, relative identifier like a flat 651 hostname, rather than a hierarchical and globally unique ADMD 652 identifier like a DNS domain name, makes configuration more difficult 653 for large sites. The hierarchical identifier permits aggregating 654 related, trusted systems together under a single, parent identifier, 655 which in turn permits assessing the trust relationship with a single 656 reference. The alternative is a flat namespace requiring 657 individually listing each trusted system. Since consumers will use 658 the identifier to determine whether to use the contents of the header 659 field: 661 o Changes to the identifier impose a large, centralized 662 administrative burden. 664 o Ongoing administrative changes require constantly updating this 665 centralized table, making it difficult to ensure that an MUA or 666 downstream filter will have access to accurate information for 667 assessing the usability of the header field's content. In 668 particular, consumers of the header field will need to know not 669 only the current identifier(s) in use but previous ones as well to 670 account for delivery latency or later re-assessment of the header 671 field's contents. 673 Examples of valid authentication identifiers are "example.com", 674 "mail.example.org", "ms1.newyork.example.com", and "example-auth". 676 2.6. Version Tokens 678 The grammar above provides for the optional inclusion of versions on 679 both the header field itself (attached to the authserv-id token) and 680 on each of the methods being reported. The method version refers to 681 the method itself, which is specified in the documents describing 682 those methods, while the authserv-id version refers to this document 683 and thus the syntax of this header field. 685 The purpose of including these is to avoid misinterpretation of the 686 results. That is, if a parser finds a version after an authserv-id 687 that it does not explicitly know, it can immediately discontinue 688 trying to parse since what follows might not be in an expected 689 format. For a method version, the parser SHOULD ignore a method 690 result if the version is not supported in case the semantics of the 691 result have a different meaning than what is expected. For example, 692 if a hypothetical DKIM version 2 yielded a "pass" result for 693 different reasons than version 1 does, a consumer of this field might 694 not want to use the altered semantics. Allowing versions in the 695 syntax is a way to indicate this and let the consumer of the header 696 field decide. 698 2.7. Defined Methods and Result Values 700 Each individual authentication method returns one of a set of 701 specific result values. The subsections below provide references to 702 the documents defining the authentication methods specifically 703 supported by this document, and their corresponding result values. 704 Verifiers SHOULD use these values as described below. New methods 705 not specified in this document, but intended to be supported by the 706 header field defined here, MUST include a similar result table either 707 in their defining documents or in supplementary ones. 709 2.7.1. DKIM and DomainKeys 711 DKIM is represented by the "dkim" method and is defined in [DKIM]. 712 DomainKeys is defined in [DOMAINKEYS] and is represented by the 713 "domainkeys" method. 715 Section 3.8 of [DOMAINKEYS] enumerates some possible results of a 716 DomainKeys evaluation. Those results are not used when generating 717 this header field; rather, the results returned are listed below. 719 A signature is "acceptable to the ADMD" if it passes local policy 720 checks (or there are no specific local policy checks). For example, 721 an ADMD policy might require that the signature(s) on the message be 722 added using the DNS domain present in the From header field of the 723 message, thus making third-party signatures unacceptable even if they 724 verify. 726 Both DKIM and DomainKeys use the same result set, as follows: 728 none: The message was not signed. 730 pass: The message was signed, the signature or signatures were 731 acceptable to the ADMD, and the signature(s) passed verification 732 tests. 734 fail: The message was signed and the signature or signatures were 735 acceptable to the ADMD, but they failed the verification test(s). 737 policy: The message was signed, but some aspect of the signature or 738 signatures was not acceptable to the ADMD. 740 neutral: The message was signed, but the signature or signatures 741 contained syntax errors or were not otherwise able to be 742 processed. This result is also used for other failures not 743 covered elsewhere in this list. 745 temperror: The message could not be verified due to some error that 746 is likely transient in nature, such as a temporary inability to 747 retrieve a public key. A later attempt may produce a final 748 result. 750 permerror: The message could not be verified due to some error that 751 is unrecoverable, such as a required header field being absent. A 752 later attempt is unlikely to produce a final result. 754 DKIM results are reported using a ptype of "header". The property, 755 however, represents one of the tags found in the DKIM-Signature 756 header field rather than a distinct header field. For example, the 757 ptype-property combination "header.d" refers to the content of the 758 "d" (signing domain) tag from within the signature header field, and 759 not a distinct header field called "d". 761 The ability to report different DKIM results for a message with 762 multiple signatures is described in [RFC6008]. 764 [DKIM] advises that if a message fails verification, it is to be 765 treated as an unsigned message. A report of "fail" here permits the 766 receiver of the report to decide how to handle the failure. A report 767 of "neutral" or "none" preempts that choice, ensuring the message 768 will be treated as if it had not been signed. 770 Section 3.1 of [DOMAINKEYS] describes a process by which the sending 771 address of the message is determined. DomainKeys results are thus 772 reported along with the signing domain name, the sending address of 773 the message, and the name of the header field from which the latter 774 was extracted. This means that a DomainKeys result includes a ptype- 775 property combination of "header.d", plus one of "header.from" and 776 "header.sender". The sending address extracted from the header is 777 included with any [MAIL]-style comments removed; moreover, the local- 778 part of the address and the "@" character are removed if it has not 779 been authenticated in some way. 781 2.7.2. SPF and Sender ID 783 SPF and Sender ID use the "spf" and "sender-id" method names, 784 respectively. The result values for SPF are defined in Section 2.6 785 of [SPF], and those definitions are included here by reference: 787 +-----------+--------------------------------+ 788 | Code | Meaning | 789 +-----------+--------------------------------+ 790 | none | [RFC7208], Section 2.6.1 | 791 +-----------+--------------------------------+ 792 | pass | [RFC7208], Section 2.6.3 | 793 +-----------+--------------------------------+ 794 | fail | [RFC7208], Section 2.6.4 | 795 +-----------+--------------------------------+ 796 | softfail | [RFC7208], Section 2.6.5 | 797 +-----------+--------------------------------+ 798 | policy | RFC 7601, Section 2.4 | 799 +-----------+--------------------------------+ 800 | neutral | [RFC7208], Section 2.6.2 | 801 +-----------+--------------------------------+ 802 | temperror | [RFC7208], Section 2.6.6 | 803 +-----------+--------------------------------+ 804 | permerror | [RFC7208], Section 2.6.7 | 805 +-----------+--------------------------------+ 807 These result codes are used in the context of this specification to 808 reflect the result returned by the component conducting SPF 809 evaluation. 811 For SPF, the ptype used is "smtp", and the property is either 812 "mailfrom" or "helo", since those values are the ones SPF can 813 evaluate. (If the SMTP client issued the EHLO command instead of 814 HELO, the property used is "helo".) 816 The "sender-id" method is described in [SENDERID]. For this method, 817 the ptype used is "header" and the property will be the name of the 818 header field from which the Purported Responsible Address (see [PRA]) 819 was extracted -- namely, one of "Resent-Sender", "Resent-From", 820 "Sender", or "From". 822 The results for Sender ID are listed and described in Section 4.2 of 823 [SENDERID], but for the purposes of this specification, the SPF 824 definitions enumerated above are used instead. Also, [SENDERID] 825 specifies result codes that use mixed case, but they are typically 826 used all lowercase in this context. 828 For both methods, an additional result of "policy" is defined, which 829 means the client was authorized to inject or relay mail on behalf of 830 the sender's DNS domain according to the authentication method's 831 algorithm, but local policy dictates that the result is unacceptable. 832 For example, "policy" might be used if SPF returns a "pass" result, 833 but a local policy check matches the sending DNS domain to one found 834 in an explicit list of unacceptable DNS domains (e.g., spammers). 836 If the retrieved sender policies used to evaluate SPF and Sender ID 837 do not contain explicit provisions for authenticating the local-part 838 (see Section 3.4.1 of [MAIL]) of an address, the "pvalue" reported 839 along with results for these mechanisms SHOULD NOT include the local- 840 part or the following "@" character. 842 2.7.3. "iprev" 844 The result values used by the "iprev" method, defined in Section 3, 845 are as follows: 847 pass: The DNS evaluation succeeded, i.e., the "reverse" and 848 "forward" lookup results were returned and were in agreement. 850 fail: The DNS evaluation failed. In particular, the "reverse" and 851 "forward" lookups each produced results, but they were not in 852 agreement, or the "forward" query completed but produced no 853 result, e.g., a DNS RCODE of 3, commonly known as NXDOMAIN, or an 854 RCODE of 0 (NOERROR) in a reply containing no answers, was 855 returned. 857 temperror: The DNS evaluation could not be completed due to some 858 error that is likely transient in nature, such as a temporary DNS 859 error, e.g., a DNS RCODE of 2, commonly known as SERVFAIL, or 860 other error condition resulted. A later attempt may produce a 861 final result. 863 permerror: The DNS evaluation could not be completed because no PTR 864 data are published for the connecting IP address, e.g., a DNS 865 RCODE of 3, commonly known as NXDOMAIN, or an RCODE of 0 (NOERROR) 866 in a reply containing no answers, was returned. This prevented 867 completion of the evaluation. A later attempt is unlikely to 868 produce a final result. 870 There is no "none" for this method since any TCP connection 871 delivering email has an IP address associated with it, so some kind 872 of evaluation will always be possible. 874 The result is reported using a ptype of "policy" (as this is not part 875 of any established protocol) and a property of "iprev". 877 For discussion of the format of DNS replies, see "Domain Names - 878 Implementation and Specification" ([DNS]). 880 2.7.4. SMTP AUTH 882 SMTP AUTH (defined in [AUTH]) is represented by the "auth" method. 883 Its result values are as follows: 885 none: SMTP authentication was not attempted. 887 pass: The SMTP client authenticated to the server reporting the 888 result using the protocol described in [AUTH]. 890 fail: The SMTP client attempted to authenticate to the server using 891 the protocol described in [AUTH] but was not successful (such as 892 providing a valid identity but an incorrect password). 894 temperror: The SMTP client attempted to authenticate using the 895 protocol described in [AUTH] but was not able to complete the 896 attempt due to some error that is likely transient in nature, such 897 as a temporary directory service lookup error. A later attempt 898 may produce a final result. 900 permerror: The SMTP client attempted to authenticate using the 901 protocol described in [AUTH] but was not able to complete the 902 attempt due to some error that is likely not transient in nature, 903 such as a permanent directory service lookup error. A later 904 attempt is not likely to produce a final result. 906 The result of AUTH is reported using a ptype of "smtp" and a property 907 of either: 909 o "auth", in which case the value is the authorization identity 910 generated by the exchange initiated by the AUTH command; or 912 o "mailfrom", in which case the value is the mailbox identified by 913 the AUTH parameter used with the MAIL FROM command. 915 If both identities are available, both can be reported. For example, 916 consider this command issued by a client that has completed session 917 authentication with the AUTH command resulting in an authorized 918 identity of "client@c.example": 920 MAIL FROM: AUTH= 922 This could result in a "resinfo" construction like so: 924 ; auth=pass smtp.auth=client@c.example smtp.mailfrom=bob@b.example 926 Note that in all cases other than "pass", the message was sent by an 927 unauthenticated client. All non-"pass" cases SHOULD thus be treated 928 as equivalent with respect to this method. 930 2.7.5. Other Registered Codes 932 Result codes were also registered in other RFCs as follows: 934 o Vouch By Reference (in [AR-VBR], represented by "vbr"); 936 o Authorized Third-Party Signatures (in [ATPS], represented by 937 "dkim-atps"); 939 o Author Domain Signing Practices (in [ADSP], represented by "dkim- 940 adsp"); 942 o Require-Recipient-Valid-Since (in [RRVS], represented by "rrvs"); 944 o S/MIME (in [SMIME-REG], represented by "smime"). 946 2.7.6. Extension Methods 948 Additional authentication method identifiers (extension methods) may 949 be defined in the future by later revisions or extensions to this 950 specification. These method identifiers are registered with the 951 Internet Assigned Numbers Authority (IANA) and, preferably, published 952 in an RFC. See Section 6 for further details. 954 Extension methods can be defined for the following reasons: 956 1. To allow additional information from new authentication systems 957 to be communicated to MUAs or downstream filters. The names of 958 such identifiers ought to reflect the name of the method being 959 defined but ought not be needlessly long. 961 2. To allow the creation of "sub-identifiers" that indicate 962 different levels of authentication and differentiate between 963 their relative strengths, e.g., "auth1-weak" and "auth1-strong". 965 Authentication method implementers are encouraged to provide adequate 966 information, via message header field comments if necessary, to allow 967 an MUA developer to understand or relay ancillary details of 968 authentication results. For example, if it might be of interest to 969 relay what data was used to perform an evaluation, such information 970 could be relayed as a comment in the header field, such as: 972 Authentication-Results: example.com; 973 foo=pass bar.baz=blob (2 of 3 tests OK) 975 Experimental method identifiers MUST only be used within ADMDs that 976 have explicitly consented to use them. These method identifiers and 977 the parameters associated with them are not documented in RFCs. 979 Therefore, they are subject to change at any time and not suitable 980 for production use. Any MTA, MUA, or downstream filter intended for 981 production use SHOULD ignore or delete any Authentication-Results 982 header field that includes an experimental (unknown) method 983 identifier. 985 2.7.7. Extension Result Codes 987 Additional result codes (extension results) might be defined in the 988 future by later revisions or extensions to this specification. 989 Result codes MUST be registered with the Internet Assigned Numbers 990 Authority (IANA) and preferably published in an RFC. See Section 6 991 for further details. 993 Experimental results MUST only be used within ADMDs that have 994 explicitly consented to use them. These results and the parameters 995 associated with them are not formally documented. Therefore, they 996 are subject to change at any time and not suitable for production 997 use. Any MTA, MUA, or downstream filter intended for production use 998 SHOULD ignore or delete any Authentication-Results header field that 999 includes an extension result. 1001 3. The "iprev" Authentication Method 1003 This section defines an additional authentication method called 1004 "iprev". 1006 "iprev" is an attempt to verify that a client appears to be valid 1007 based on some DNS queries, which is to say that the IP address is 1008 explicitly associated with a domain name. Upon receiving a session 1009 initiation of some kind from a client, the IP address of the client 1010 peer is queried for matching names (i.e., a number-to-name 1011 translation, also known as a "reverse lookup" or a "PTR" record 1012 query). Once that result is acquired, a lookup of each of the names 1013 (i.e., a name-to-number translation, or an "A" or "AAAA" record 1014 query) thus retrieved is done. The response to this second check 1015 will typically result in at least one mapping back to the client's IP 1016 address. 1018 Expressed as an algorithm: If the client peer's IP address is I, the 1019 list of names to which I maps (after a "PTR" query) is the set N, and 1020 the union of IP addresses to which each member of N maps (after 1021 corresponding "A" and "AAAA" queries) is L, then this test is 1022 successful if I is an element of L. 1024 Often an MTA receiving a connection that fails this test will simply 1025 reject the connection using the enhanced status code defined in 1027 [AUTH-ESC]. If an operator instead wishes to make this information 1028 available to downstream agents as a factor in handling decisions, it 1029 records a result in accordance with Section 2.7.3. 1031 The response to a PTR query could contain multiple names. To prevent 1032 heavy DNS loads, agents performing these queries MUST be implemented 1033 such that the number of names evaluated by generation of 1034 corresponding A or AAAA queries is limited so as not to be unduly 1035 taxing to the DNS infrastructure, though it MAY be configurable by an 1036 administrator. As an example, Section 4.6.4 of [SPF] chose a limit 1037 of 10 for its implementation of this algorithm. 1039 "DNS Extensions to Support IP Version 6" ([DNS-IP6]) discusses the 1040 query formats for the IPv6 case. 1042 There is some contention regarding the wisdom and reliability of this 1043 test. For example, in some regions, it can be difficult for this 1044 test ever to pass because the practice of arranging to match the 1045 forward and reverse DNS is infrequently observed. Therefore, the 1046 precise implementation details of how a verifier performs an "iprev" 1047 test are not specified here. The verifier MAY report a successful or 1048 failed "iprev" test at its discretion having done some kind of check 1049 of the validity of the connection's identity using DNS. It is 1050 incumbent upon an agent making use of the reported "iprev" result to 1051 understand what exactly that particular verifier is attempting to 1052 report. 1054 Extensive discussion of reverse DNS mapping and its implications can 1055 be found in "Considerations for the use of DNS Reverse Mapping" 1056 ([DNSOP-REVERSE]). In particular, it recommends that applications 1057 avoid using this test as a means of authentication or security. Its 1058 presence in this document is not an endorsement but is merely 1059 acknowledgment that the method remains common and provides the means 1060 to relay the results of that test. 1062 4. Adding the Header Field to a Message 1064 This specification makes no attempt to evaluate the relative 1065 strengths of various message authentication methods that may become 1066 available. The methods listed are an order-independent set; their 1067 sequence does not indicate relative strength or importance of one 1068 method over another. Instead, the MUA or downstream filter consuming 1069 this header field is to interpret the result of each method based on 1070 its own knowledge of what that method evaluates. 1072 Each "method" MUST refer to an authentication method declared in the 1073 IANA registry or an extension method as described in Section 2.7.6, 1074 and each "result" MUST refer to a result code declared in the IANA 1075 registry or an extension result code as defined in Section 2.7.7. 1076 See Section 6 for further information about the registered methods 1077 and result codes. 1079 An MTA compliant with this specification adds this header field 1080 (after performing one or more message authentication tests) to 1081 indicate which MTA or ADMD performed the test, which test got 1082 applied, and what the result was. If an MTA applies more than one 1083 such test, it adds this header field either once per test or once 1084 indicating all of the results. An MTA MUST NOT add a result to an 1085 existing header field. 1087 An MTA MAY add this header field containing only the authentication 1088 identifier portion and the "none" token (see Section 2.2) to indicate 1089 explicitly that no message authentication schemes were applied prior 1090 to delivery of this message. 1092 An MTA adding this header field has to take steps to identify it as 1093 legitimate to the MUAs or downstream filters that will ultimately 1094 consume its content. One process to do so is described in Section 5. 1095 Further measures may be necessary in some environments. Some 1096 possible solutions are enumerated in Section 7.1. This document does 1097 not mandate any specific solution to this issue as each environment 1098 has its own facilities and limitations. 1100 Most known message authentication methods focus on a particular 1101 identifier to evaluate. SPF and Sender ID differ in that they can 1102 yield a result based on more than one identifier; specifically, SPF 1103 can evaluate the RFC5321.HELO parameter or the RFC5321.MailFrom 1104 parameter, and Sender ID can evaluate the RFC5321.MailFrom parameter 1105 or the Purported Responsible Address (PRA) identity. When generating 1106 this field to report those results, only the parameter that yielded 1107 the result is included. 1109 For MTAs that add this header field, adding header fields in order 1110 (at the top), per Section 3.6 of [MAIL], is particularly important. 1111 Moreover, this header field SHOULD be inserted above any other trace 1112 header fields such MTAs might prepend. This placement allows easy 1113 detection of header fields that can be trusted. 1115 End users making direct use of this header field might inadvertently 1116 trust information that has not been properly vetted. If, for 1117 example, a basic SPF result were to be relayed that claims an 1118 authenticated addr-spec, the local-part of that addr-spec has 1119 actually not been authenticated. Thus, an MTA adding this header 1120 field SHOULD NOT include any data that has not been authenticated by 1121 the method(s) being applied. Moreover, MUAs SHOULD NOT render to 1122 users such information if it is presented by a method known not to 1123 authenticate it. 1125 4.1. Header Field Position and Interpretation 1127 In order to ensure non-ambiguous results and avoid the impact of 1128 false header fields, MUAs and downstream filters SHOULD NOT interpret 1129 this header field unless specifically configured to do so by the user 1130 or administrator. That is, this interpretation should not be "on by 1131 default". Naturally then, users or administrators ought not activate 1132 such a feature unless (1) they are certain the header field will be 1133 validly added by an agent within the ADMD that accepts the mail that 1134 is ultimately read by the MUA, and (2) instances of the header field 1135 that appear to originate within the ADMD but are actually added by 1136 foreign MTAs will be removed before delivery. 1138 Furthermore, MUAs and downstream filters SHOULD NOT interpret this 1139 header field unless the authentication service identifier it bears 1140 appears to be one used within its own ADMD as configured by the user 1141 or administrator. 1143 MUAs and downstream filters MUST ignore any result reported using a 1144 "result" not specified in the IANA "Result Code" registry or a 1145 "ptype" not listed in the "Email Authentication Property Types" 1146 registry for such values as defined in Section 6. Moreover, such 1147 agents MUST ignore a result indicated for any "method" they do not 1148 specifically support. 1150 An MUA SHOULD NOT reveal these results to end users, absent careful 1151 human factors design considerations and testing, for the presentation 1152 of trust-related materials. For example, an attacker could register 1153 examp1e.com (note the digit "1" (one)) and send signed mail to 1154 intended victims; a verifier would detect that the signature was 1155 valid and report a "pass" even though it's clear the DNS domain name 1156 was intended to mislead. See Section 7.2 for further discussion. 1158 As stated in Section 2.1, this header field MUST be treated as though 1159 it were a trace header field as defined in Section 3.6.7 of [MAIL] 1160 and hence MUST NOT be reordered and MUST be prepended to the message, 1161 so that there is generally some indication upon delivery of where in 1162 the chain of handling MTAs the message authentication was done. 1164 Note that there are a few message handlers that are only capable of 1165 appending new header fields to a message. Strictly speaking, these 1166 handlers are not compliant with this specification. They can still 1167 add the header field to carry authentication details, but any signal 1168 about where in the handling chain the work was done may be lost. 1169 Consumers SHOULD be designed such that this can be tolerated, 1170 especially from a producer known to have this limitation. 1172 MUAs SHOULD ignore instances of this header field discovered within 1173 message/rfc822 MIME attachments. 1175 Further discussion of these topics can be found in Section 7 below. 1177 4.2. Local Policy Enforcement 1179 Some sites have a local policy that considers any particular 1180 authentication policy's non-recoverable failure results (typically 1181 "fail" or similar) as justification for rejecting the message. In 1182 such cases, the border MTA SHOULD issue an SMTP rejection response to 1183 the message, rather than adding this header field and allowing the 1184 message to proceed toward delivery. This is more desirable than 1185 allowing the message to reach an internal host's MTA or spam filter, 1186 thus possibly generating a local rejection such as a Delivery Status 1187 Notification (DSN) [DSN] to a forged originator. Such generated 1188 rejections are colloquially known as "backscatter". 1190 The same MAY also be done for local policy decisions overriding the 1191 results of the authentication methods (e.g., the "policy" result 1192 codes described in Section 2.7). 1194 Such rejections at the SMTP protocol level are not possible if local 1195 policy is enforced at the MUA and not the MTA. 1197 5. Removing Existing Header Fields 1199 For security reasons, any MTA conforming to this specification MUST 1200 delete any discovered instance of this header field that claims, by 1201 virtue of its authentication service identifier, to have been added 1202 within its trust boundary but that did not come directly from another 1203 trusted MTA. For example, an MTA for example.com receiving a message 1204 MUST delete or otherwise obscure any instance of this header field 1205 bearing an authentication service identifier indicating that the 1206 header field was added within example.com prior to adding its own 1207 header fields. This could mean each MTA will have to be equipped 1208 with a list of internal MTAs known to be compliant (and hence 1209 trustworthy). 1211 For simplicity and maximum security, a border MTA could remove all 1212 instances of this header field on mail crossing into its trust 1213 boundary. However, this may conflict with the desire to access 1214 authentication results performed by trusted external service 1215 providers. It may also invalidate signed messages whose signatures 1216 cover external instances of this header field. A more robust border 1217 MTA could allow a specific list of authenticating MTAs whose 1218 information is to be admitted, removing the header field originating 1219 from all others. 1221 As stated in Section 1.2, a formal definition of "trust boundary" is 1222 deliberately not made here. It is entirely possible that a border 1223 MTA for example.com will explicitly trust authentication results 1224 asserted by upstream host example.net even though they exist in 1225 completely disjoint administrative boundaries. In that case, the 1226 border MTA MAY elect not to delete those results; moreover, the 1227 upstream host doing some authentication work could apply a signing 1228 technology such as [DKIM] on its own results to assure downstream 1229 hosts of their authenticity. An example of this is provided in 1230 Appendix B. 1232 Similarly, in the case of messages signed using [DKIM] or other 1233 message-signing methods that sign header fields, this removal action 1234 could invalidate one or more signatures on the message if they 1235 covered the header field to be removed. This behavior can be 1236 desirable since there's little value in validating the signature on a 1237 message with forged header fields. However, signing agents MAY 1238 therefore elect to omit these header fields from signing to avoid 1239 this situation. 1241 An MTA SHOULD remove any instance of this header field bearing a 1242 version (express or implied) that it does not support. However, an 1243 MTA MUST remove such a header field if the [SMTP] connection relaying 1244 the message is not from a trusted internal MTA. This means the MTA 1245 needs to be able to understand versions of this header field at least 1246 as late as the ones understood by the MUAs or other consumers within 1247 its ADMD. 1249 6. IANA Considerations 1251 IANA has registered the defined header field and created registries 1252 as described below. These registry actions were originally defined 1253 by [RFC5451] and updated by [RFC6577] and [RFC7001]. The created 1254 registries were further updated in [RFC7601] to make them more 1255 complete. 1257 Each is listed below, though generally they are not changed by this 1258 document. 1260 6.1. The Authentication-Results Header Field 1262 The Authentication-Results header field was added to the IANA 1263 "Permanent Message Header Field Names" registry, per the procedure 1264 found in [IANA-HEADERS]. That entry will be updated to reference 1265 this document. The following is the registration template: 1267 Header field name: Authentication-Results 1268 Applicable protocol: mail ([MAIL]) 1269 Status: Standard 1270 Author/Change controller: IETF 1271 Specification document(s): [this document] 1272 Related information: none 1274 6.2. "Email Authentication Methods" Registry Description 1276 No changes are made to this registry. 1278 6.3. "Email Authentication Methods" Registry Update 1280 No changes are made to this registry. 1282 6.4. "Email Authentication Property Types" Registry 1284 [RFC7410] created the "Email Authentication Property Types" registry. 1286 No changes are made to this registry. However, it should be noted 1287 that Section 2.3 contains slightly different language than prior 1288 versions of this document, allowing a broader space from which to 1289 extract meaningful identifiers and report them through this 1290 mechanism. 1292 6.5. "Email Authentication Result Names" Description 1294 No changes are made to this registry. 1296 6.6. "Email Authentication Result Names" Update 1298 No changes are made to this registry. 1300 7. Security Considerations 1302 The following security considerations apply when adding or processing 1303 the Authentication-Results header field: 1305 7.1. Forged Header Fields 1307 An MUA or filter that accesses a mailbox whose messages are handled 1308 by a non-conformant MTA, and understands Authentication-Results 1309 header fields, could potentially make false conclusions based on 1310 forged header fields. A malicious user or agent could forge a header 1311 field using the DNS domain of a receiving ADMD as the authserv-id 1312 token in the value of the header field and, with the rest of the 1313 value, claim that the message was properly authenticated. The non- 1314 conformant MTA would fail to strip the forged header field, and the 1315 MUA could inappropriately trust it. 1317 For this reason, it is best not to have processing of the 1318 Authentication-Results header field enabled by default; instead, it 1319 should be ignored, at least for the purposes of enacting filtering 1320 decisions, unless specifically enabled by the user or administrator 1321 after verifying that the border MTA is compliant. It is acceptable 1322 to have an MUA aware of this specification but have an explicit list 1323 of hostnames whose Authentication-Results header fields are 1324 trustworthy; however, this list should initially be empty. 1326 Proposed alternative solutions to this problem were made some time 1327 ago and are listed below. To date, they have not been developed due 1328 to lack of demand but are documented here should the information be 1329 useful at some point in the future: 1331 1. Possibly the simplest is a digital signature protecting the 1332 header field, such as using [DKIM], that can be verified by an 1333 MUA by using a posted public key. Although one of the main 1334 purposes of this document is to relieve the burden of doing 1335 message authentication work at the MUA, this only requires that 1336 the MUA learn a single authentication scheme even if a number of 1337 them are in use at the border MTA. Note that [DKIM] requires 1338 that the From header field be signed, although in this 1339 application, the signing agent (a trusted MTA) likely cannot 1340 authenticate that value, so the fact that it is signed should be 1341 ignored. Where the authserv-id is the ADMD's domain name, the 1342 authserv-id matching this valid internal signature's "d=" DKIM 1343 value is sufficient. 1345 2. Another would be a means to interrogate the MTA that added the 1346 header field to see if it is actually providing any message 1347 authentication services and saw the message in question, but this 1348 isn't especially palatable given the work required to craft and 1349 implement such a scheme. 1351 3. Yet another might be a method to interrogate the internal MTAs 1352 that apparently handled the message (based on Received header 1353 fields) to determine whether any of them conform to Section 5 of 1354 this memo. This, too, has potentially high barriers to entry. 1356 4. Extensions to [IMAP], [SMTP], and [POP3] could be defined to 1357 allow an MUA or filtering agent to acquire the authserv-id in use 1358 within an ADMD, thus allowing it to identify which 1359 Authentication-Results header fields it can trust. 1361 5. On the presumption that internal MTAs are fully compliant with 1362 Section 3.6 of [MAIL] and the compliant internal MTAs are using 1363 their own hostnames or the ADMD's DNS domain name as the 1364 authserv-id token, the header field proposed here should always 1365 appear above a Received header added by a trusted MTA. This can 1366 be used as a test for header field validity. 1368 Support for some of these is being considered for future work. 1370 In any case, a mechanism needs to exist for an MUA or filter to 1371 verify that the host that appears to have added the header field (a) 1372 actually did so and (b) is legitimately adding that header field for 1373 this delivery. Given the variety of messaging environments deployed 1374 today, consensus appears to be that specifying a particular mechanism 1375 for doing so is not appropriate for this document. 1377 Mitigation of the forged header field attack can also be accomplished 1378 by moving the authentication results data into metadata associated 1379 with the message. In particular, an [SMTP] extension could be 1380 established to communicate authentication results from the border MTA 1381 to intermediate and delivery MTAs; the latter of these could arrange 1382 to store the authentication results as metadata retrieved and 1383 rendered along with the message by an [IMAP] client aware of a 1384 similar extension in that protocol. The delivery MTA would be told 1385 to trust data via this extension only from MTAs it trusts, and border 1386 MTAs would not accept data via this extension from any source. There 1387 is no vector in such an arrangement for forgery of authentication 1388 data by an outside agent. 1390 7.2. Misleading Results 1392 Until some form of service for querying the reputation of a sending 1393 agent is widely deployed, the existence of this header field 1394 indicating a "pass" does not render the message trustworthy. It is 1395 possible for an arriving piece of spam or other undesirable mail to 1396 pass checks by several of the methods enumerated above (e.g., a piece 1397 of spam signed using [DKIM] by the originator of the spam, which 1398 might be a spammer or a compromised system). In particular, this 1399 issue is not resolved by forged header field removal discussed above. 1401 Hence, MUAs and downstream filters must take some care with use of 1402 this header even after possibly malicious headers are scrubbed. 1404 7.3. Header Field Position 1406 Despite the requirements of [MAIL], header fields can sometimes be 1407 reordered en route by intermediate MTAs. The goal of requiring 1408 header field addition only at the top of a message is an 1409 acknowledgment that some MTAs do reorder header fields, but most do 1410 not. Thus, in the general case, there will be some indication of 1411 which MTAs (if any) handled the message after the addition of the 1412 header field defined here. 1414 7.4. Reverse IP Query Denial-of-Service Attacks 1416 Section 4.6.4 of [SPF] describes a DNS-based denial-of-service attack 1417 for verifiers that attempt DNS-based identity verification of 1418 arriving client connections. A verifier wishing to do this check and 1419 report this information needs to take care not to go to unbounded 1420 lengths to resolve "A" and "PTR" queries. MUAs or other filters 1421 making use of an "iprev" result specified by this document need to be 1422 aware of the algorithm used by the verifier reporting the result and, 1423 especially, its limitations. 1425 7.5. Mitigation of Backscatter 1427 Failing to follow the instructions of Section 4.2 can result in a 1428 denial-of-service attack caused by the generation of [DSN] messages 1429 (or equivalent) to addresses that did not send the messages being 1430 rejected. 1432 7.6. Internal MTA Lists 1434 Section 5 describes a procedure for scrubbing header fields that may 1435 contain forged authentication results about a message. A compliant 1436 installation will have to include, at each MTA, a list of other MTAs 1437 known to be compliant and trustworthy. Failing to keep this list 1438 current as internal infrastructure changes may expose an ADMD to 1439 attack. 1441 7.7. Attacks against Authentication Methods 1443 If an attack becomes known against an authentication method, clearly 1444 then the agent verifying that method can be fooled into thinking an 1445 inauthentic message is authentic, and thus the value of this header 1446 field can be misleading. It follows that any attack against the 1447 authentication methods supported by this document is also a security 1448 consideration here. 1450 7.8. Intentionally Malformed Header Fields 1452 It is possible for an attacker to add an Authentication-Results 1453 header field that is extraordinarily large or otherwise malformed in 1454 an attempt to discover or exploit weaknesses in header field parsing 1455 code. Implementers must thoroughly verify all such header fields 1456 received from MTAs and be robust against intentionally as well as 1457 unintentionally malformed header fields. 1459 7.9. Compromised Internal Hosts 1461 An internal MUA or MTA that has been compromised could generate mail 1462 with a forged From header field and a forged Authentication-Results 1463 header field that endorses it. Although it is clearly a larger 1464 concern to have compromised internal machines than it is to prove the 1465 value of this header field, this risk can be mitigated by arranging 1466 that internal MTAs will remove this header field if it claims to have 1467 been added by a trusted border MTA (as described above), yet the 1468 [SMTP] connection is not coming from an internal machine known to be 1469 running an authorized MTA. However, in such a configuration, 1470 legitimate MTAs will have to add this header field when legitimate 1471 internal-only messages are generated. This is also covered in 1472 Section 5. 1474 7.10. Encapsulated Instances 1476 MIME messages can contain attachments of type "message/rfc822", which 1477 contain other messages. Such an encapsulated message can also 1478 contain an Authentication-Results header field. Although the 1479 processing of these is outside of the intended scope of this document 1480 (see Section 1.3), some early guidance to MUA developers is 1481 appropriate here. 1483 Since MTAs are unlikely to strip Authentication-Results header fields 1484 after mailbox delivery, MUAs are advised in Section 4.1 to ignore 1485 such instances within MIME attachments. Moreover, when extracting a 1486 message digest to separate mail store messages or other media, such 1487 header fields should be removed so that they will never be 1488 interpreted improperly by MUAs that might later consume them. 1490 7.11. Reverse Mapping 1492 Although Section 3 of this memo includes explicit support for the 1493 "iprev" method, its value as an authentication mechanism is limited. 1494 Implementers of both this proposal and agents that use the data it 1495 relays are encouraged to become familiar with the issues raised by 1496 [DNSOP-REVERSE] when deciding whether or not to include support for 1497 "iprev". 1499 8. References 1501 8.1. Normative References 1503 [ABNF] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax 1504 Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/ 1505 RFC5234, January 2008, 1506 . 1508 [IANA-HEADERS] 1509 Klyne, G., Nottingham, M., and J. Mogul, "Registration 1510 Procedures for Message Header Fields", BCP 90, RFC 3864, 1511 DOI 10.17487/RFC3864, September 2004, 1512 . 1514 [KEYWORDS] 1515 Bradner, S., "Key words for use in RFCs to Indicate 1516 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 1517 RFC2119, March 1997, 1518 . 1520 [MAIL] Resnick, P., Ed., "Internet Message Format", RFC 5322, 1521 DOI 10.17487/RFC5322, October 2008, 1522 . 1524 [MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 1525 Extensions (MIME) Part One: Format of Internet Message 1526 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 1527 . 1529 [RFC5451] Kucherawy, M., "Message Header Field for Indicating 1530 Message Authentication Status", RFC 5451, DOI 10.17487/ 1531 RFC5451, April 2009, 1532 . 1534 [RFC6008] Kucherawy, M., "Authentication-Results Registration for 1535 Differentiating among Cryptographic Results", RFC 6008, 1536 DOI 10.17487/RFC6008, September 2010, 1537 . 1539 [RFC6577] Kucherawy, M., "Authentication-Results Registration Update 1540 for Sender Policy Framework (SPF) Results", RFC 6577, 1541 DOI 10.17487/RFC6577, March 2012, 1542 . 1544 [RFC7001] Kucherawy, M., "Message Header Field for Indicating 1545 Message Authentication Status", RFC 7001, DOI 10.17487/ 1546 RFC7001, September 2013, 1547 . 1549 [RFC7601] Kucherawy, M., "Message Header Field for Indicating 1550 Message Authentication Status", RFC 7601, DOI 10.17487/ 1551 RFC7601, August 2015, 1552 . 1554 [SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 1555 DOI 10.17487/RFC5321, October 2008, 1556 . 1558 8.2. Informative References 1560 [ADSP] Allman, E., Fenton, J., Delany, M., and J. Levine, 1561 "DomainKeys Identified Mail (DKIM) Author Domain Signing 1562 Practices (ADSP)", RFC 5617, DOI 10.17487/RFC5617, 1563 August 2009, . 1565 [AR-VBR] Kucherawy, M., "Authentication-Results Registration for 1566 Vouch by Reference Results", RFC 6212, DOI 10.17487/ 1567 RFC6212, April 2011, 1568 . 1570 [ATPS] Kucherawy, M., "DomainKeys Identified Mail (DKIM) 1571 Authorized Third-Party Signatures", RFC 6541, 1572 DOI 10.17487/RFC6541, February 2012, 1573 . 1575 [AUTH] Siemborski, R., Ed. and A. Melnikov, Ed., "SMTP Service 1576 Extension for Authentication", RFC 4954, DOI 10.17487/ 1577 RFC4954, July 2007, 1578 . 1580 [AUTH-ESC] 1581 Kucherawy, M., "Email Authentication Status Codes", 1582 RFC 7372, DOI 10.17487/RFC7372, September 2014, 1583 . 1585 [DKIM] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., 1586 "DomainKeys Identified Mail (DKIM) Signatures", STD 76, 1587 RFC 6376, DOI 10.17487/RFC6376, September 2011, 1588 . 1590 [DMARC] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based 1591 Message Authentication, Reporting, and Conformance 1592 (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015, 1593 . 1595 [DNS] Mockapetris, P., "Domain names - implementation and 1596 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1597 November 1987, . 1599 [DNS-IP6] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 1600 "DNS Extensions to Support IP Version 6", RFC 3596, 1601 DOI 10.17487/RFC3596, October 2003, 1602 . 1604 [DNSOP-REVERSE] 1605 Senie, D. and A. Sullivan, "Considerations for the use of 1606 DNS Reverse Mapping", Work in Progress, draft-ietf-dnsop- 1607 reverse-mapping-considerations-06, March 2008. 1609 [DOMAINKEYS] 1610 Delany, M., "Domain-Based Email Authentication Using 1611 Public Keys Advertised in the DNS (DomainKeys)", RFC 4870, 1612 DOI 10.17487/RFC4870, May 2007, 1613 . 1615 [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format 1616 for Delivery Status Notifications", RFC 3464, 1617 DOI 10.17487/RFC3464, January 2003, 1618 . 1620 [EMAIL-ARCH] 1621 Crocker, D., "Internet Mail Architecture", RFC 5598, 1622 DOI 10.17487/RFC5598, July 2009, 1623 . 1625 [IMAP] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 1626 4rev1", RFC 3501, DOI 10.17487/RFC3501, March 2003, 1627 . 1629 [POP3] Myers, J. and M. Rose, "Post Office Protocol - Version 3", 1630 STD 53, RFC 1939, DOI 10.17487/RFC1939, May 1996, 1631 . 1633 [PRA] Lyon, J., "Purported Responsible Address in E-Mail 1634 Messages", RFC 4407, DOI 10.17487/RFC4407, April 2006, 1635 . 1637 [RFC7410] Kucherawy, M., "A Property Types Registry for the 1638 Authentication-Results Header Field", RFC 7410, 1639 DOI 10.17487/RFC7410, December 2014, 1640 . 1642 [RRVS] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid- 1643 Since Header Field and SMTP Service Extension", RFC 7293, 1644 DOI 10.17487/RFC7293, July 2014, 1645 . 1647 [SECURITY] 1648 Rescorla, E. and B. Korver, "Guidelines for Writing RFC 1649 Text on Security Considerations", BCP 72, RFC 3552, 1650 DOI 10.17487/RFC3552, July 2003, 1651 . 1653 [SENDERID] 1654 Lyon, J. and M. Wong, "Sender ID: Authenticating E-Mail", 1655 RFC 4406, DOI 10.17487/RFC4406, April 2006, 1656 . 1658 [SMIME-REG] 1659 Melnikov, A., "Authentication-Results Registration for 1660 S/MIME Signature Verification", RFC 7281, DOI 10.17487/ 1661 RFC7281, June 2014, 1662 . 1664 [SPF] Kitterman, S., "Sender Policy Framework (SPF) for 1665 Authorizing Use of Domains in Email, Version 1", RFC 7208, 1666 DOI 10.17487/RFC7208, April 2014, 1667 . 1669 [VBR] Hoffman, P., Levine, J., and A. Hathcock, "Vouch By 1670 Reference", RFC 5518, DOI 10.17487/RFC5518, April 2009, 1671 . 1673 Appendix A. Legacy MUAs 1675 Implementers of this protocol should be aware that many MUAs are 1676 unlikely to be retrofitted to support the new header field and its 1677 semantics. In the interests of convenience and quicker adoption, a 1678 delivery MTA might want to consider adding things that are processed 1679 by existing MUAs in addition to the Authentication-Results header 1680 field. One suggestion is to include a Priority header field, on 1681 messages that don't already have such a header field, containing a 1682 value that reflects the strength of the authentication that was 1683 accomplished, e.g., "low" for weak or no authentication, "normal" or 1684 "high" for good or strong authentication. 1686 Some modern MUAs can already filter based on the content of this 1687 header field. However, there is keen interest in having MUAs make 1688 some kind of graphical representation of this header field's meaning 1689 to end users. Until this capability is added (i.e., while this 1690 proposal and its successors are being adopted), other interim means 1691 of conveying authentication results may be necessary. 1693 Appendix B. Authentication-Results Examples 1695 This section presents some examples of the use of this header field 1696 to indicate authentication results. 1698 B.1. Trivial Case; Header Field Not Present 1700 The trivial case: 1702 Received: from mail-router.example.com 1703 (mail-router.example.com [192.0.2.1]) 1704 by server.example.org (8.11.6/8.11.6) 1705 with ESMTP id g1G0r1kA003489; 1706 Fri, Feb 15 2002 17:19:07 -0800 1707 From: sender@example.com 1708 Date: Fri, Feb 15 2002 16:54:30 -0800 1709 To: receiver@example.org 1710 Message-Id: <12345.abc@example.com> 1711 Subject: here's a sample 1713 Hello! Goodbye! 1715 Example 1: Trivial Case 1717 The Authentication-Results header field is completely absent. The 1718 MUA may make no conclusion about the validity of the message. This 1719 could be the case because the message authentication services were 1720 not available at the time of delivery, or no service is provided, or 1721 the MTA is not in compliance with this specification. 1723 B.2. Nearly Trivial Case; Service Provided, but No Authentication Done 1725 A message that was delivered by an MTA that conforms to this 1726 specification but provides no actual message authentication service: 1728 Authentication-Results: example.org 1; none 1729 Received: from mail-router.example.com 1730 (mail-router.example.com [192.0.2.1]) 1731 by server.example.org (8.11.6/8.11.6) 1732 with ESMTP id g1G0r1kA003489; 1733 Fri, Feb 15 2002 17:19:07 -0800 1734 From: sender@example.com 1735 Date: Fri, Feb 15 2002 16:54:30 -0800 1736 To: receiver@example.org 1737 Message-Id: <12345.abc@example.com> 1738 Subject: here's a sample 1740 Hello! Goodbye! 1742 Example 2: Header Present but No Authentication Done 1744 The Authentication-Results header field is present, showing that the 1745 delivering MTA conforms to this specification. It used its DNS 1746 domain name as the authserv-id. The presence of "none" (and the 1747 absence of any method or result tokens) indicates that no message 1748 authentication was done. The version number of the specification to 1749 which the field's content conforms is explicitly provided. 1751 B.3. Service Provided, Authentication Done 1753 A message that was delivered by an MTA that conforms to this 1754 specification and applied some message authentication: 1756 Authentication-Results: example.com; 1757 spf=pass smtp.mailfrom=example.net 1758 Received: from dialup-1-2-3-4.example.net 1759 (dialup-1-2-3-4.example.net [192.0.2.200]) 1760 by mail-router.example.com (8.11.6/8.11.6) 1761 with ESMTP id g1G0r1kA003489; 1762 Fri, Feb 15 2002 17:19:07 -0800 1763 From: sender@example.net 1764 Date: Fri, Feb 15 2002 16:54:30 -0800 1765 To: receiver@example.com 1766 Message-Id: <12345.abc@example.net> 1767 Subject: here's a sample 1769 Hello! Goodbye! 1771 Example 3: Header Reporting Results 1773 The Authentication-Results header field is present, indicating that 1774 the border MTA conforms to this specification. The authserv-id is 1775 once again the DNS domain name. Furthermore, the message was 1776 authenticated by that MTA via the method specified in [SPF]. Note 1777 that since that method cannot authenticate the local-part, it has 1778 been omitted from the result's value. The MUA could extract and 1779 relay this extra information if desired. 1781 B.4. Service Provided, Several Authentications Done, Single MTA 1783 A message that was relayed inbound via a single MTA that conforms to 1784 this specification and applied three different message authentication 1785 checks: 1787 Authentication-Results: example.com; 1788 auth=pass (cram-md5) smtp.auth=sender@example.net; 1789 spf=pass smtp.mailfrom=example.net 1790 Authentication-Results: example.com; 1791 sender-id=pass header.from=example.net 1792 Received: from dialup-1-2-3-4.example.net (8.11.6/8.11.6) 1793 (dialup-1-2-3-4.example.net [192.0.2.200]) 1794 by mail-router.example.com (8.11.6/8.11.6) 1795 with ESMTPA id g1G0r1kA003489; 1796 Fri, Feb 15 2002 17:19:07 -0800 1797 Date: Fri, Feb 15 2002 16:54:30 -0800 1798 To: receiver@example.com 1799 From: sender@example.net 1800 Message-Id: <12345.abc@example.net> 1801 Subject: here's a sample 1803 Hello! Goodbye! 1805 Example 4: Headers Reporting Results from One MTA 1807 The Authentication-Results header field is present, indicating that 1808 the delivering MTA conforms to this specification. Once again, the 1809 receiving DNS domain name is used as the authserv-id. Furthermore, 1810 the sender authenticated herself/himself to the MTA via a method 1811 specified in [AUTH], and both SPF and Sender ID checks were done and 1812 passed. The MUA could extract and relay this extra information if 1813 desired. 1815 Two Authentication-Results header fields are not required since the 1816 same host did all of the checking. The authenticating agent could 1817 have consolidated all the results into one header field. 1819 This example illustrates a scenario in which a remote user on a 1820 dial-up connection (example.net) sends mail to a border MTA 1821 (example.com) using SMTP authentication to prove identity. The 1822 dial-up provider has been explicitly authorized to relay mail as 1823 example.com, producing "pass" results from the SPF and Sender ID 1824 checks. 1826 B.5. Service Provided, Several Authentications Done, Different MTAs 1828 A message that was relayed inbound by two different MTAs that conform 1829 to this specification and applied multiple message authentication 1830 checks: 1832 Authentication-Results: example.com; 1833 sender-id=fail header.from=example.com; 1834 dkim=pass (good signature) header.d=example.com 1835 Received: from mail-router.example.com 1836 (mail-router.example.com [192.0.2.1]) 1837 by auth-checker.example.com (8.11.6/8.11.6) 1838 with ESMTP id i7PK0sH7021929; 1839 Fri, Feb 15 2002 17:19:22 -0800 1840 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; d=example.com; 1841 t=1188964191; c=simple/simple; h=From:Date:To:Subject: 1842 Message-Id:Authentication-Results; 1843 bh=sEuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m70; 1844 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 1845 Authentication-Results: example.com; 1846 auth=pass (cram-md5) smtp.auth=sender@example.com; 1847 spf=fail smtp.mailfrom=example.com 1848 Received: from dialup-1-2-3-4.example.net 1849 (dialup-1-2-3-4.example.net [192.0.2.200]) 1850 by mail-router.example.com (8.11.6/8.11.6) 1851 with ESMTPA id g1G0r1kA003489; 1852 Fri, Feb 15 2002 17:19:07 -0800 1853 From: sender@example.com 1854 Date: Fri, Feb 15 2002 16:54:30 -0800 1855 To: receiver@example.com 1856 Message-Id: <12345.abc@example.com> 1857 Subject: here's a sample 1859 Hello! Goodbye! 1861 Example 5: Headers Reporting Results from Multiple MTAs 1863 The Authentication-Results header field is present, indicating 1864 conformance to this specification. Once again, the authserv-id used 1865 is the recipient's DNS domain name. The header field is present 1866 twice because two different MTAs in the chain of delivery did 1867 authentication tests. The first MTA, mail-router.example.com, 1868 reports that SMTP AUTH and SPF were both used and that the former 1869 passed while the latter failed. In the SMTP AUTH case, additional 1870 information is provided in the comment field, which the MUA can 1871 choose to render if desired. 1873 The second MTA, auth-checker.example.com, reports that it did a 1874 Sender ID test (which failed) and a DKIM test (which passed). Again, 1875 additional data about one of the tests is provided as a comment, 1876 which the MUA may choose to render. Also noteworthy here is the fact 1877 that there is a DKIM signature added by example.com that assured the 1878 integrity of the lower Authentication-Results field. 1880 Since different hosts did the two sets of authentication checks, the 1881 header fields cannot be consolidated in this example. 1883 This example illustrates more typical transmission of mail into 1884 example.com from a user on a dial-up connection example.net. The 1885 user appears to be legitimate as he/she had a valid password allowing 1886 authentication at the border MTA using SMTP AUTH. The SPF and Sender 1887 ID tests failed since example.com has not granted example.net 1888 authority to relay mail on its behalf. However, the DKIM test passed 1889 because the sending user had a private key matching one of 1890 example.com's published public keys and used it to sign the message. 1892 B.6. Service Provided, Multi-tiered Authentication Done 1894 A message that had authentication done at various stages, one of 1895 which was outside the receiving ADMD: 1897 Authentication-Results: example.com; 1898 dkim=pass reason="good signature" 1899 header.i=@mail-router.example.net; 1900 dkim=fail reason="bad signature" 1901 header.i=@newyork.example.com 1902 Received: from mail-router.example.net 1903 (mail-router.example.net [192.0.2.250]) 1904 by chicago.example.com (8.11.6/8.11.6) 1905 for 1906 with ESMTP id i7PK0sH7021929; 1907 Fri, Feb 15 2002 17:19:22 -0800 1908 DKIM-Signature: v=1; a=rsa-sha256; s=furble; 1909 d=mail-router.example.net; t=1188964198; c=relaxed/simple; 1910 h=From:Date:To:Message-Id:Subject:Authentication-Results; 1911 bh=ftA9J6GtX8OpwUECzHnCkRzKw1uk6FNiLfJl5Nmv49E=; 1912 b=oINEO8hgn/gnunsg ... 9n9ODSNFSDij3= 1913 Authentication-Results: example.net; 1914 dkim=pass (good signature) header.i=@newyork.example.com 1915 Received: from smtp.newyork.example.com 1916 (smtp.newyork.example.com [192.0.2.220]) 1917 by mail-router.example.net (8.11.6/8.11.6) 1918 with ESMTP id g1G0r1kA003489; 1919 Fri, Feb 15 2002 17:19:07 -0800 1920 DKIM-Signature: v=1; a=rsa-sha256; s=gatsby; 1921 d=newyork.example.com; 1922 t=1188964191; c=simple/simple; 1923 h=From:Date:To:Message-Id:Subject; 1924 bh=sEu28nfs9fuZGD/pSr7ANysbY3jtdaQ3Xv9xPQtS0m7=; 1925 b=EToRSuvUfQVP3Bkz ... rTB0t0gYnBVCM= 1926 From: sender@newyork.example.com 1927 Date: Fri, Feb 15 2002 16:54:30 -0800 1928 To: meetings@example.net 1929 Message-Id: <12345.abc@newyork.example.com> 1930 Subject: here's a sample 1932 Example 6: Headers Reporting Results from Multiple MTAs in Different 1933 ADMDs 1935 In this example, we see multi-tiered authentication with an extended 1936 trust boundary. 1938 The message was sent from someone at example.com's New York office 1939 (newyork.example.com) to a mailing list managed at an intermediary. 1941 The message was signed at the origin using DKIM. 1943 The message was sent to a mailing list service provider called 1944 example.net, which is used by example.com. There, 1945 meetings@example.net is expanded to a long list of recipients, one of 1946 whom is at the Chicago office. In this example, we will assume that 1947 the trust boundary for chicago.example.com includes the mailing list 1948 server at example.net. 1950 The mailing list server there first authenticated the message and 1951 affixed an Authentication-Results header field indicating such using 1952 its DNS domain name for the authserv-id. It then altered the message 1953 by affixing some footer text to the body, including some 1954 administrivia such as unsubscription instructions. Finally, the 1955 mailing list server affixes a second DKIM signature and begins 1956 distribution of the message. 1958 The border MTA for chicago.example.com explicitly trusts results from 1959 mail-router.example.net, so that header field is not removed. It 1960 performs evaluation of both signatures and determines that the first 1961 (most recent) is a "pass" but, because of the aforementioned 1962 modifications, the second is a "fail". However, the first signature 1963 included the Authentication-Results header added at mail- 1964 router.example.net that validated the second signature. Thus, 1965 indirectly, it can be determined that the authentications claimed by 1966 both signatures are indeed valid. 1968 Note that two styles of presenting metadata about the result are in 1969 use here. In one case, the "reason=" clause is present, which is 1970 intended for easy extraction by parsers; in the other case, the CFWS 1971 production of the ABNF is used to include such data as a header field 1972 comment. The latter can be harder for parsers to extract given the 1973 varied supported syntaxes of mail header fields. 1975 B.7. Comment-Heavy Example 1977 The formal syntax permits comments within the content in a number of 1978 places. For the sake of illustration, this example is also legal: 1980 Authentication-Results: foo.example.net (foobar) 1 (baz); 1981 dkim (Because I like it) / 1 (One yay) = (wait for it) fail 1982 policy (A dot can go here) . (like that) expired 1983 (this surprised me) = (as I wasn't expecting it) 1362471462 1985 Example 7: A Very Comment-Heavy but Perfectly Legal Example 1987 Appendix C. Operational Considerations about Message Authentication 1989 This protocol is predicated on the idea that authentication (and 1990 presumably in the future, reputation) work is typically done by 1991 border MTAs rather than MUAs or intermediate MTAs; the latter merely 1992 make use of the results determined by the former. Certainly this is 1993 not mandatory for participation in electronic mail or message 1994 authentication, but this protocol and its deployment to date are 1995 based on that model. The assumption satisfies several common ADMD 1996 requirements: 1998 1. Service operators prefer to resolve the handling of problem 1999 messages as close to the border of the ADMD as possible. This 2000 enables, for example, rejection of messages at the SMTP level 2001 rather than generating a DSN internally. Thus, doing any of the 2002 authentication or reputation work exclusively at the MUA or 2003 intermediate MTA renders this desire unattainable. 2005 2. Border MTAs are more likely to have direct access to external 2006 sources of authentication or reputation information since modern 2007 MUAs are more likely to be heavily firewalled. Thus, some MUAs 2008 might not even be able to complete the task of performing 2009 authentication or reputation evaluations without complex proxy 2010 configurations or similar burdens. 2012 3. MUAs rely upon the upstream MTAs within their trust boundaries to 2013 make correct (as much as is possible) evaluations about the 2014 message's envelope, header, and content. Thus, MUAs don't need 2015 to know how to do the work that upstream MTAs do; they only need 2016 the results of that work. 2018 4. Evaluations about the quality of a message, from simple token 2019 matching (e.g., a list of preferred DNS domains) to cryptanalysis 2020 (e.g., public/private key work), do have a cost and thus need to 2021 be minimized. To that end, performing those tests at the border 2022 MTA is far preferred to doing that work at each MUA that handles 2023 a message. If an ADMD's environment adheres to common messaging 2024 protocols, a reputation query or an authentication check 2025 performed by a border MTA would return the same result as the 2026 same query performed by an MUA. By contrast, in an environment 2027 where the MUA does the work, a message arriving for multiple 2028 recipients would thus cause authentication or reputation 2029 evaluation to be done more than once for the same message (i.e., 2030 at each MUA), causing needless amplification of resource use and 2031 creating a possible denial-of-service attack vector. 2033 5. Minimizing change is good. As new authentication and reputation 2034 methods emerge, the list of methods supported by this header 2035 field would presumably be extended. If MUAs simply consume the 2036 contents of this header field rather than actually attempt to do 2037 authentication and/or reputation work, then MUAs only need to 2038 learn to parse this header field once; emergence of new methods 2039 requires only a configuration change at the MUAs and software 2040 changes at the MTAs (which are presumably fewer in number). When 2041 choosing to implement these functions in MTAs vs. MUAs, the 2042 issues of individual flexibility, infrastructure inertia, and 2043 scale of effort must be considered. It is typically easier to 2044 change a single MUA than an MTA because the modification affects 2045 fewer users and can be pursued with less care. However, changing 2046 many MUAs is more effort than changing a smaller number of MTAs. 2048 6. For decisions affecting message delivery and display, assessment 2049 based on authentication and reputation is best performed close to 2050 the time of message transit, as a message makes its journey 2051 toward a user's inbox, not afterwards. DKIM keys and IP address 2052 reputations, etc., can change over time or even become invalid, 2053 and users can take a long time to read a message once delivered. 2054 The value of this work thus degrades, perhaps quickly, once the 2055 delivery process has completed. This seriously diminishes the 2056 value of this work when done elsewhere than at MTAs. 2058 Many operational choices are possible within an ADMD, including the 2059 venue for performing authentication and/or reputation assessment. 2060 The current specification does not dictate any of those choices. 2061 Rather, it facilitates those cases in which information produced by 2062 one stage of analysis needs to be transported with the message to the 2063 next stage. 2065 Appendix D. Changes since RFC 7001 2067 o Applied RFC 7410. 2069 o Updated all references to RFC 4408 with RFC 7208. 2071 o Added section explaining "property" values. (Addressed Erratum 2072 #4201.) 2074 o Did some minor text reorganization. 2076 o Gave registry history -- enough that this is now the authoritative 2077 registry definition. 2079 o Added text explaining each of the method-ptype-property tuples 2080 registered by this document. 2082 o Changed the meaning of the "Defined" column of the methods 2083 registry to be the place where each entry was created and 2084 described; it is expected that this will then refer to the 2085 method's defining document. Provided IANA with corresponding 2086 update instructions. 2088 o Cleaned up registry structure and content, and replaced all 2089 references to RFC 7001 with pointers to this document. 2091 o Added references: [DMARC], [PRA], [RFC6008], [RFC6577], [RRVS], 2092 [SMIME-REG]. 2094 o Added description of values that can be extracted from SMTP AUTH 2095 sessions and an example. 2097 o Provided much more complete descriptions of reporting DomainKeys 2098 results. 2100 o Added more detail about Sender ID. 2102 o Marked all ADSP and DomainKeys entries as deprecated since their 2103 defining documents are as well. 2105 o Reworked some text around ignoring unknown ptypes. 2107 o Completely described the ptypes registry. 2109 o Mentioned that EHLO is mapped to HELO for SPF. 2111 o RFC 7208 uses all-lowercase result strings now, so adjusted prose 2112 accordingly. 2114 o Updated list of supported methods, and mentioned the registries 2115 immediately below. 2117 o Mentioned that when a local-part is removed, the "@" goes with it. 2119 o Referred to RFC 7328 in the "iprev" definition. 2121 o Corrected the "smime-part" prose. 2123 o Updated examples that use SMTP AUTH to claim "with ESMTPA" in the 2124 Received fields. 2126 o Made minor editorial adjustments. 2128 Appendix E. Acknowledgments 2130 The author wishes to acknowledge the following individuals for their 2131 review and constructive criticism of this document: TBD 2133 Author's Address 2135 Murray S. Kucherawy 2136 270 Upland Drive 2137 San Francisco, CA 94127 2138 United States 2140 Email: superuser@gmail.com